1. Field of the Invention
The present invention generally relates to solar cell modules. More specifically, the invention relates to an improved solar cell module that is inexpensive and lightweight, as well as a method of making the same.
2. Description of Related Art
With the ready availability of solar energy in outer space for a spacecraft such as a satellite, the conversion of solar energy into electrical energy with photovoltaic cells is an obvious choice for producing power. Solar energy is also a major consideration for producing electrical power in terrestrial applications, as costs associated with more traditional power plants increase yearly. Higher efficiency in power conversion of sunlight to electricity equates to either lighter weight spacecraft or higher payload capacity, both of which have monetary benefit. Higher efficiency in terrestrial systems equates to higher system efficiency, which reduces balance of system costs, such as land area, support structures and wiring. One method for increasing efficiency is to manufacture solar cells with multiple junctions, or layers having different energy band gaps which have been stacked so that each cell or layer can absorb a different part of the wide energy distribution in the sunlight. Because of the high voltage of these cells compared to silicon and their susceptibility to reverse bias breakdown, there is a requirement to protect each cell with a bypass diode. Attachment of the diode to each cell is in addition to attaching interconnects for the purpose of increasing voltage in a solar cell circuit by series connection.
Past connection of cells, however, have involved multiple interconnects and diode tabs. The diode tab has commonly been a separate strip of metal, which has been wrapped around from the top to rear sides of the cell. This has required much handling, attaching, and cleaning operations, which increase the cost of manufacturing the solar module and results in solar cell attrition due to handling.
Some interconnection methods have used monolithic, metallized wraparound or wrapthrough areas, which allow access to both positive and negative cell polarities on the rear of the cell. This method involves evaporating the metal wraparound or wrapthrough in an evaporation chamber. A disadvantage to this metal wraparound has been the associated cost of lasing or micro-blasting a via for the metal, etching and photolithographic steps required to monolithically attach and insulate the metal wraparound to the cell. Another disadvantage to the wraparound process is the tendency to shunt the cell through defects in the thin dielectric used to insulate the evaporated metal wraparound.
Traditionally, once the individual solar cells have been interconnected in a string, the string has been bonded to a 2-facesheet honeycomb substrate. Wiring the cell strings together in series for higher voltage or in parallel for higher current has typically been accomplished by the use of insulated wire and soldered joints. However, this method of soldering involves a time consuming set of manual processes which require inspection, rework and cleaning. Along with being time consuming, those steps also lead to attrition of the fragile and expensive solar cells.
Furthermore, solar cell panels must be designed robust to survive the rigors of the space environment. The individual solar cells and their substrate can be subject to significant mechanical vibration during a launch and thermal cycling during the course of the spacecraft's mission in space. The thermal cycling, in turn, leads to thermal expansion and contraction of the various materials. This can cause stress on the components of the solar cell panel if there is a coefficient of expansion (CTE) mismatch between the component materials. With greater stress in terms of frequency and magnitude, there can be a shorter life expectancy of the panel. Ultimately, the spacecraft on which the solar panel is used will have a shorter life and result in greater costs to replace it.
Past designs of solar cell panels that attempt to address one or more of the above performance and manufacturing issues have been numerous, including U.S. Pat. No. 4,083,097. Therein, a method of making encapsulated solar cell modules includes a polymer cover film that is molded to provide an embossed surface having depressions arranged in a row. Each depression has the same configuration as a solar cell. Solar cells with positive and negative contacts on the back surface are preferred and can be positioned in the depressions with the front surfaces of the cells that face the light source contacting the bottom of the depressions. A second polymer film having interconnecting circuitry metallization is placed over the back surfaces of the cells so that the cells are electrically connected. A disadvantage, however, is the lack of direct bonding between the back surfaces of the cells and the second polymer film, which leads to a greater potential for separation from the metallization. Another disadvantage is that the device may not work in a severe thermal environment, such as outer space where thermal expansion will result in a loss of electrical connection.
A solar panel using a printed circuit substrate is shown in U.S. Pat. No. 5,185,042. Solar cells are physically and electrically connected to a substrate via interconnect pads. Positive and negative terminals on the back side of the cells are preferably connected by soldering to the interconnect pads. If the terminals are on opposite sides of the cells, metallic interconnectors may be used to connect terminals on the tops sides, over the cell edges, and to the interconnect pads, even though the specific fashion is not described. An adhesive may optionally secure the cells to the substrate, although, again, the particular manner is not described. Stress relief loops bound the interconnect pads to electrical traces encapsulated in the substrate. This results in the solar cells being effectively mounted to the substrate on coiled springs. On the back side of the substrate, electrically conductive mounting pads enable attachment to elements such as blocking and shunting diodes. If the cell is soldered to the spring shaped conductor then the solder could bridge across the spring, thus making it lose its advantage as an absorber of thermal stresses. Another disadvantage is that the configuration of a coiled loop provides a relatively weak structure that is susceptible to structural failure when stressed and, thus, electrical connection failure. Yet another disadvantage is that this design requires either a wrapthrough metal configuration to bring both cell contacts to the rear side of the solar cells or a tab. The tab type described in the patent bridges off the cell onto an adjacent conductive pad, which increases the area required for a solar array of a given power design. The wrapthrough metal configuration has disadvantages, which have been described above.
As can be seen, there is a need for an improved solar cell module and method of making the same. A further need is for a solar cell module that is lightweight, yet inexpensive to manufacture. Another need is for a solar cell module that provides a low cost, top-to-rear side connection of cells having a bypass diode. Yet another need is for a method of making solar cell panels which reduces the need for handling, attaching, and cleaning operations to facilitate automation. A method of making solar cell panels is needed, which also eliminates the need for lasing or micro-blasting, etching and photolithographic steps otherwise required to monolithically attach and insulate a metal wraparound to the cell. An apparatus and method is further needed that minimizes the potential of shunting cells through defects in the dielectric that insulates the metal wraparound from the cell substrate.